Add support for foreign keys to composite primary keys

[edu-zamora]

Using @PrimaryKey annotation in multiple fields of a class will create a composite primary key, but is there any way to create foreign key to point to that composite primary key?

As far as I know, we can only add a @foreignkey annotation to a specific field, which will only point to a part of the composite primary key, and therefore, fail.

Another alternative would be to, instead of creating a composite primary key, use @AutoIncrementPrimaryKey and add a multiple column unique constraint to the columns that were included in the composite primary key.

Unfortunately, I don't see a way to do that either. I opened an issue here:
#55

For now, what would be the best way to ensure for a table that the combination of two columns is unique?

Could I add that constraint manually in the table (using migration.addRawStatement?)?

Thanks a lot for your work, I am enjoying the library so far! :)

EDIT:
It seems that constraints cannot be added after table creation with sqlite:
http://stackoverflow.com/questions/1884818/how-do-i-add-a-foreign-key-to-an-existing-sqlite-3-6-21-table